Energy conservation in heavy-hydrocarbon distillation

ABSTRACT

An aromatics complex producing one or more xylene isomers offers a large number of opportunities to conserve energy by heat exchange within the complex. One previously unrecognized opportunity is through providing two parallel distillation columns operating at different pressures to separate C 8  aromatics from C 9 + aromatics. The parallel columns offer additional opportunities to conserve energy within the complex through heat exchange in associated xylene recovery facilities.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation-in-Part of copending U.S. applicationSer. No. 12/868,309 filed Aug. 25, 2010, the contents of which arehereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to an improved process for energy savings in thedistillation of hydrocarbons. More specifically, the present inventionconcerns energy conservation within an aromatics-processing complexproducing xylene isomers.

BACKGROUND OF THE INVENTION

The xylene isomers are produced in large volumes from petroleum asfeedstocks for a variety of important industrial chemicals. The mostimportant of the xylene isomers is para-xylene, the principal feedstockfor polyester, which continues to enjoy a high growth rate from largebase demand. Ortho-xylene is used to produce phthalic anhydride, whichsupplies high-volume but relatively mature markets. Meta-xylene is usedin lesser but growing volumes for such products as plasticizers, azodyes and wood preservers. Ethylbenzene generally is present in xylenemixtures and is occasionally recovered for styrene production, but isusually considered a less-desirable component of C₈ aromatics.

Among the aromatic hydrocarbons, the overall importance of xylenesrivals that of benzene as a feedstock for industrial chemicals. Xylenesand benzene are produced from petroleum by reforming naphtha but not insufficient volume to meet demand, thus conversion of other hydrocarbonsis necessary to increase the yield of xylenes and benzene. Often tolueneis de-alkylated to produce benzene or selectively disproportionated toyield benzene and C₈ aromatics from which the individual xylene isomersare recovered.

An aromatics complex flow scheme has been disclosed by Meyers in theHandbook of Petroleum Refining Processes, 2d. Edition in 1997 byMcGraw-Hill, and is incorporated herein by reference.

Aromatics complexes producing xylenes are substantial consumers ofenergy, notably in distillation operations to prepare feedstocks andseparate products from conversion processes. The separation of xylenesfrom heavy aromatics in particular offers substantial potential forenergy savings. Energy conservation in such processes would not onlyreduce processing costs but also would address current concerns aboutcarbon emissions.

SUMMARY OF THE INVENTION

A broad embodiment of the present invention comprises a distillationprocess comprising two or more xylene columns separating C₈-aromaticsstreams from a C₉-and-heavier aromatics streams contained in at leastone lower-boiling and at least one higher-boiling feed stream, whereinthe at least one higher-boiling feed stream has a higher content ofC₉-and-heavier aromatics than the at least one lower-boiling feedstream, comprising distilling the at least one lower-boiling feed streamin at least one first xylene column at an elevated pressure to separatea first C₈-aromatics streams from a first C₉-and-heavier aromaticsstream, distilling the at least one higher-boiling feed stream in atleast one second xylene column at a low pressure column to separate asecond C₈-aromatics stream from a second C₉-and-heavier aromaticsstream, and condensing an overhead stream from the at least one firstcolumn by exchanging heat with one or both of a reboiler of the secondcolumn and a steam generator.

A more specific embodiment comprises a distillation process comprisingtwo xylene columns separating C₈-aromatics streams from C₉-and-heavieraromatics streams contained in at least one lower-boiling and at leastone higher-boiling feed stream, wherein the at least one higher-boilingfeed stream has a higher content of C₉-and-heavier aromatics than the atleast one lower-boiling feed stream, comprising distilling the at leastone lower-boiling feed stream in at least one first xylene column at anelevated pressure to separate a first C₈-aromatics streams from a firstC₉-and-heavier aromatics stream, distilling the at least onehigher-boiling feed stream in at least one second xylene column at a lowpressure column to separate a second C₈-aromatics stream from a secondC₉-and-heavier aromatics stream, and condensing an overhead stream fromthe at least one first column by exchanging heat with a reboiler of thesecond column.

An alternative embodiment comprises a distillation process comprisingtwo xylene columns separating C₈-aromatics streams from C₉-and-heavieraromatics streams contained in at least one lower-boiling and at leastone higher-boiling feed stream, wherein the at least one higher-boilingfeed stream has a higher content of C₉-and-heavier aromatics than the atleast one lower-boiling feed stream, comprising distilling the at leastone lower-boiling feed stream in at least one first xylene column at anelevated pressure to separate a first C₈-aromatics streams from a firstC₉-and-heavier aromatics stream, distilling the at least onehigher-boiling feed stream in at least one second xylene column at a lowpressure column to separate a second C₈-aromatics stream from a secondC₉-and-heavier aromatics stream, and condensing an overhead stream fromthe at least one first column

Additional objects, embodiments and details of this invention can beobtained and inferred from the following detailed description of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an aromatics complex in whichenergy-savings concepts could be applied.

FIG. 2 illustrates an aromatics complex in which energy conservation isapplied.

FIG. 3 shows the application of energy conservation in the distillationof C₈ aromatics from heavy aromatics.

FIG. 4 illustrates examples of specific units within an aromaticscomplex in which direct heat exchange could achieve energy savings.

FIG. 5 illustrates an aromatics complex in which some of theenergy-savings concepts described herein are applied as a supplement orsubstitute for other energy savings.

FIG. 6 illustrates the generation of steam from specific units within anaromatics complex.

DETAILED DESCRIPTION OF THE INVENTION

The feedstream to the present process generally comprises alkylaromatichydrocarbons of the general formula C₆H_((6-n))R_(n), where n is aninteger from 0 to 5 and each R may be CH₃, C₂H₅, C₃H₇, or C₄H₉, in anycombination. The aromatics-rich feed stream to the process of theinvention may be derived from a variety of sources, including withoutlimitation catalytic reforming, steam pyrolysis of naphtha, distillatesor other hydrocarbons to yield light olefins and heavier aromatics-richbyproducts (including gasoline-range material often referred to as“pygas”), and catalytic or thermal cracking of distillates and heavyoils to yield products in the gasoline range. Products from pyrolysis orother cracking operations generally will be hydrotreated according toprocesses well known in the industry before being charged to the complexin order to remove sulfur, olefins and other compounds which wouldaffect product quality and/or damage catalysts or adsorbents employedtherein. Light cycle oil from catalytic cracking also may bebeneficially hydrotreated and/or hydrocracked according to knowntechnology to yield products in the gasoline range; the hydrotreatingpreferably also includes catalytic reforming to yield the aromatics-richfeed stream. If the feed stream is catalytic reformate, the reformerpreferably is operated at high severity to achieve high aromatics yieldwith a low concentration of nonaromatics in the product.

FIG. 1 is a simplified flow diagram of a typical aromatics-processingcomplex of the known art directed to the production of at least onexylene isomer. The complex may process an aromatics-rich feed which hasbeen derived, for example, from catalytic reforming. Usually such astream will have been treated to remove olefinic compounds and lightends, e.g., butanes and lighter hydrocarbons and preferably pentanes;such removal, however, is not essential to the practice of the broadaspects of this invention. The aromatics-containing feed stream containsbenzene, toluene and C₈ aromatics and typically contains higheraromatics and aliphatic hydrocarbons including naphthenes.

The feed stream is passed via conduit 10 via a heat exchanger 12 toreformate splitter 14 and distilled to separate a stream comprising C₈and heavier aromatics, withdrawn as a bottoms stream in conduit 16, fromtoluene and lighter hydrocarbons recovered overhead via conduit 18. Thetoluene and lighter hydrocarbons are sent to extractive distillationprocess unit 20 which separates a largely aliphatic raffinate in conduit21 from a benzene-toluene aromatics stream in conduit 22. The aromaticsstream in conduit 22 is separated, along with stripped transalkylationproduct in conduit 45 and overhead from para-xylene finishing column inconduit 57, in benzene column 23 into a benzene stream in conduit 24 anda toluene-and-heavier aromatics stream in conduit 25 which is sent to atoluene column 26. Toluene is recovered overhead from this column inconduit 27 and may be sent partially or totally to a transalkylationunit 40 as shown and discussed hereinafter.

A bottoms stream from the toluene column 26 is passed via conduit 28,along with bottoms from the reformate splitter in conduit 16, aftertreating via clay treater 17, and recycle C₈ aromatics in conduit 65, toxylene column 30. The fractionator 30 separates concentrated C₈aromatics as overhead in conduit 31 from a high-boiling streamcomprising C₉, C₁₀ and heavier aromatics as a bottoms stream in conduit32. This bottoms stream is passed in conduit 32 to heavy-aromaticscolumn 70. The heavies column provides an overhead stream in conduit 71containing C₉ and at least some of the C₁₀ aromatics, with higherboiling compounds, primarily C₁₁ and higher alkylaromatics, beingwithdrawn as a bottoms stream via conduit 72.

The C₉+ aromatics from heavies column in conduit 71 is combined with thetoluene-containing overhead contained in conduit 27 as feed totransalkylation reactor 40, which contains a transalkylation catalyst asknown in the art to produce a transalkylation product comprising benzenethrough C₁₁+ aromatics with xylenes as the focus. The transalkylationproduct in conduit 41 is stripped in stripper 42 to remove gases inconduit 43 and C₆ and lighter hydrocarbons which are returned viaconduit 44 to extractive distillation 20 for recovery of light aromaticsand purification of benzene. Bottoms from the stripper are sent inconduit 45 to benzene column 23 to recover benzene product andunconverted toluene.

The C₈-aromatics overhead provided by fractionator 30 containspara-xylene, meta-xylene, ortho-xylene and ethylbenzene and passes viaconduit 31 to para-xylene separation process 50. The separation processoperates, preferably via moving-bed adsorption using a desorbent, toprovide a mixture of para-xylene and desorbent via conduit 51 to extractcolumn 52, which separates para-xylene via conduit 53 from returneddesorbent in conduit 54; the para-xylene is purified in finishing column55, yielding a para-xylene product via conduit 56 and light materialwhich is returned to benzene column 23 via conduit 57. A non-equilibriummixture of C₈-aromatics raffinate and desorbent from the separationprocess 50 is sent via conduit 58 to raffinate column 59, whichseparates a raffinate for isomerization in conduit 60 from returneddesorbent in conduit 61.

The raffinate, comprising a non-equilibrium mixture of xylene isomersand ethylbenzene, is sent via conduit 60 to isomerization reactor 62.The raffinate is isomerized in reactor 62, which contains anisomerization catalyst to provide a product approaching equilibriumconcentrations of C₈-aromatic isomers. The product is passed via conduit63 to deheptanizer 64, which removes C₇ and lighter hydrocarbons withbottoms passing via conduit 65 to xylene column 30 to separate C₉ andheavier materials from the isomerized C₈-aromatics. Overhead liquid fromdeheptanizer 64 is sent to stripper 66, which removes light materialsoverhead in conduit 67 from C₆ and C₇ materials which are sent viaconduit 68 to the extractive distillation unit 20 for recovery ofbenzene and toluene values.

There are many possible variations of this scheme within the known art,as the skilled routineer will recognize. For example, the entire C₆-C₈reformate or only the benzene-containing portion may be subjected toextraction. Para-xylene may be recovered from a C₈-aromatic mixture bycrystallization rather than adsorption. Meta-xylene as well aspara-xylene may be recovered from a C₈-aromatic mixture by adsorption,and ortho-xylene may be recovered by fractionation. Alternatively, theC₉-and heavier stream or the heavy-aromatics stream is processed usingsolvent extraction or solvent distillation with a polar solvent orstripping with steam or other media to separate highly condensedaromatics as a residual stream from C₉+ recycle to transalkylation. Insome cases, the entire heavy-aromatic stream may be processed directlyin the transalkylation unit. The present invention is useful in theseand other variants of an aromatics-processing scheme, aspects of whichare described in U.S. Pat. No. 6,740,788 which is incorporated herein byreference.

The separation of C₈ aromatics from heavy aromatics in fractionator 30is a situation in which the distillation process of the inventiongenerally is effective. A distillation process of the present inventionis represented by two or more xylene columns each effectingsubstantially the same separation between C₈ and C₉+ aromatics containedin two or more internal or external-feed streams of the aromaticscomplex designated respectively as a first and a second feed streams.Preferably the two streams comprise a first feed stream which ishigher-boiling and a second feed stream which is lower-boiling, whereinthe higher-boiling first feed stream has a higher content of C₉+hydrocarbons than the second feed stream. The invention comprisesdistilling the first feed stream in at least one first fractionationcolumn at a low pressure to separate a first C₈-aromatics stream from afirst C₉-and-heavier aromatics stream, distilling the second feed streamin a second fractionation column at an elevated pressure to separate asecond C₈-aromatics stream from a second C₉-and-heavier aromaticsstream, and circulating an overhead stream from the second column toprovide heat to a reboiler of the first column. The low pressuretypically is between 100 and 800 kPa and the elevated pressure is chosento enable heat transfer from the first column to the second andtypically is at least about 400 kPa above the low pressure. This conceptof different pressures in parallel columns is particularly valuable whenthe heavy components present in the higher-boiling feed stream aresubject to degradation at reboiler temperatures needed to separate thelight and heavy components.

The second fractionation column processes a second feed stream with alower concentration of heavy materials subject to decomposition than thefeed to the first column, and the pressure thus may be raised higher inorder to effect energy savings through heat exchange between the firstand second columns without loss of product yield or risk of equipmentfouling. This feed preferably comprises most or all of the isomerized C₈aromatics from the isomerization reactor following deheptanization, butmay also comprise other C₈-aromatic streams with low concentrations ofheavy aromatics. This stream to the second column typically containsless than about 10 weight-% C₉+ aromatics, more often less than about 5weight-% C₉+ aromatics, and frequently less than about 2 weight-% C₉+aromatics. Effectively, the process comprises operating the secondcolumn at a pressure that would enable the overhead to provide heat to areboiler of the first column and, preferably, a reboiler of at least oneother column and/or steam generator in an associated processing complex.

In another embodiment, the process comprises operating the secondfractionation column at a pressure that would enable the overhead toprovide heat to generate steam useful in an associated processingcomplex. Further, the C₈-aromatics fractionator may comprise three ormore columns comprising additional heat exchange between overheads andreboilers in an analogous manner to the above description.

FIG. 2 is an energy-efficient aromatics complex employing a number ofconcepts of the invention. For ease of reference, a parallel numberingsystem is employed to those of FIGS. 1 and 2. The feed stream is passedvia conduit 110 via heat exchangers 112 and 113, which raise thetemperature of the feed stream, to reformate splitter 114. The heatexchange is supplied via conduits 212 and 213 respectively from the netpara-xylene product and the recovered desorbent from the para-xyleneseparation process as discussed later in this section.

As in FIG. 1, C₈ and heavier aromatics are withdrawn as a bottoms streamin conduit 116 while toluene and lighter hydrocarbons recovered overheadvia conduit 118 are sent to extractive distillation process unit 120which separates a largely aliphatic raffinate in conduit 121 from abenzene-toluene aromatics stream in conduit 122. The aromatics stream inconduit 122 is separated, along with stripped transalkylation product inconduit 145 and overhead from para-xylene finishing column in conduit157, in fractionator 123 into a benzene stream in conduit 124 and atoluene-and-heavier aromatics stream in conduit 125 which is sent to atoluene column 126. Toluene is recovered overhead from this column inconduit 127 and may be sent partially or totally to a transalkylationunit 140 as shown and discussed hereinafter.

A bottoms stream from the toluene column 126 is passed via conduit 128,along with bottoms from the reformate splitter in conduit 116, aftertreating via clay treater 117, and a purge stream of heavy aromatics inconduit 138, to low-pressure first xylene column 130. The feed stream tothis column is characterized as a higher-boiling feed stream, as itgenerally contains more than about 5 weight-% C₉+ aromatics and oftenmore than about 10 weight-% C₉+ aromatics. Other C8-aromatics streamshaving significant contents of C₉ and heavier aromatics, includingstreams obtained from sources outside the complex, also may be added tothis higher-boiling feed stream; a portion of deheptanizer bottoms instream 165 also may be included depending on overall energy balances.The low-pressure xylene column separates concentrated first C₈-aromaticsstream as overhead in conduit 131 from a high-boiling firstC₉-and-heavier stream comprising C₉, C₁₀ and heavier aromatics as abottoms stream in conduit 132.

Simultaneously, an isomerized C₈-aromatics stream is passed via conduit165 to a high-pressure second xylene column 133. This is characterizedas a lower-boiling feed stream which contains a lower concentration ofheavy materials subject to decomposition than the feed to column 130,and the column pressure thus can be increased in order to effect energysavings. Other C₈-aromatics-containing streams having similarly lowcontents of C₉-and-heavier aromatics, including streams obtained fromsources outside the complex, also may be contained in the feed stream tothis column. The second xylene column separates a second C₈-aromaticsstream as overhead in conduit 134 from a second C₉-and-heavier stream inconduit 132. At least a portion of overhead vapor from the high-pressurexylene column in conduit 134 preferably is employed to reboillow-pressure xylene column 130 in reboiler 135, leaving as a condensedliquid to the xylene-separation process 150 in conduit 136 as well asreflux (not shown) to column 133. In addition, the overhead in conduit134 may be used to provide energy to the reboiler of extract column 152or other such services which are described later or will be apparent tothe skilled routineer.

The C₉+ bottoms stream passing to reboiler 137 may provide energy viaone or both of the stream before the reboiler in conduit 270 and theheated stream from the reboiler in conduit 259 for reboilingrespectively one or both of heavy-aromatics column 170 and raffinatecolumn 159; the bottoms stream after heat exchange would be sent to theheavy-aromatics column 170. Other similar heat-exchange services will beapparent to the skilled routineer. The net bottoms stream in conduit 138usually is passed through column 130 or may be in conduit 139 combineddirectly with the stream in conduit 132 to heavies column 170. Theheavies column provides an overhead a stream in conduit 171 containingC₉ and at least some of the C₁₀ aromatics, with higher boilingcompounds, primarily C₁₁ and higher alkylaromatics, being withdrawn as abottoms stream via conduit 172. This column may be reboiled by xylenecolumn bottoms in conduit 270, as discussed above. Overhead vapor fromcolumns 130 and 170 also may generate steam respectively via conduits230 and 271 as indicated, with condensed liquids either serving asreflux to each column or as net overhead respectively in streams 131 or171.

The C₉+ aromatics from heavies column in conduit 171 is combined withthe toluene-containing overhead contained in conduit 127 as feed totransalkylation reactor 140 to produce a transalkylation productcontaining xylenes. The transalkylation product in conduit 141 isstripped in stripper 142 to remove gases in conduit 143 and C₇ andlighter liquids which are returned via conduit 144 to extractivedistillation 120 for recovery of light aromatics following stabilizationin isomerate stripper 166. Bottoms from the stripper are sent in conduit145 to benzene column 123 to recover benzene product and unconvertedtoluene.

The first and second C₈-aromatics streams provided by xylene columns 130and 133, containing para-xylene, meta-xylene, ortho-xylene andethylbenzene, pass via conduit 131 and 136 to xylene-isomer separationprocess 150. The description herein may be applicable to the recovery ofone or more xylene isomers other than para-xylene; however, thedescription is presented for para-xylene for ease of understanding. Theseparation process operates via a moving-bed adsorption process toprovide a first mixture of para-xylene and desorbent via conduit 151 toextract column 152, which separates para-xylene via conduit 153 fromreturned desorbent in conduit 154. Extract column 152 preferably isoperated at an elevated pressure, at least about 300 kPa and morepreferably about 500 kPa or higher, such that the overhead from thecolumn is at sufficient temperature to reboil finishing column 155 viaconduit 256 or deheptanizer 164 via conduit 265. Heat supplied forreboiling duty via conduits 256 and 265 results in the condensation ofthe extract in these streams which is either or both refluxed to column152 (not shown) or sent as a net stream in conduit 153 to finishingcolumn 155. The para-xylene is purified in finishing column 155,yielding a para-xylene product via conduit 156 and light material whichis returned to benzene column 123 via conduit 157.

A second mixture of raffinate, as a non-equilibrium blend of C₈aromatics, and desorbent from separation process 150 is sent via conduit158 to raffinate column 159, which separates a raffinate toisomerization in conduit 160 from returned desorbent in conduit 161. Theraffinate column may be operated at higher pressure to generate steamvia conduit 260 or to exchange heat in other areas of the complex;condensed liquids from such heat exchange either serve as reflux to theraffinate column or as net overhead in conduit 160. Recovered desorbentin conduits 154 and 161 and net finishing column bottoms may heat theincoming feed stream in conduit 110 via conduits 213 and 212,respectively.

The raffinate, comprising a non-equilibrium blend of xylene isomers andethylbenzene, is sent via conduit 160 to isomerization reactor 162. Inthe isomerization reactor 162, raffinate is isomerized to provide aproduct approaching equilibrium concentrations of C₈-aromatic isomers.The product is passed via conduit 163 to deheptanizer 164, which removesC₇ and lighter hydrocarbons and preferably is reboiled using overhead inconduit 265 from extract column 152. Bottoms from the deheptanizerpasses via conduit 165 to xylene column 133 to separate C₉ and heaviermaterials from the isomerized C₈-aromatics. Overhead liquid fromdeheptanizer 164 is sent to stripper 166, which separates lightmaterials overhead in conduit 167 from C₆ and C₇ materials which aresent via conduit 168 to the extractive distillation unit 120 forrecovery and purification of benzene and toluene values. Pressures ofdeheptanizer 164 and stripper 166 are selected to exchange heat orgenerate steam in a manner analogous to the xylene columns discussedelsewhere in this specification.

FIG. 3 shows in more detail the heat exchange of the invention betweenparallel xylene distillation columns 130 and 133. Feed to thelow-pressure xylene column 130 comprises bottoms from the toluene columnvia conduit 128, clay-treated bottoms from the reformate splitter inconduit 116, and purge C₈ aromatics in conduit 138 and may compriseother C₈-aromatics-containing streams not suitable for processing in thehigh-pressure xylene column as well as a portion of the deheptanizedstream 165 if appropriate for energy balances. The combined feeds ofheavy reformate and toluene-column bottoms may contain heavy aromaticswhich are susceptible to degradation at high temperatures, and operatingat a pressure lower than 800 kPa permits temperatures to be maintainedin the bottom of the column and reboiler which avoid such decomposition.The low-pressure xylene column separates concentrated C₈ aromatics asoverhead in conduit 131 from a high-boiling stream comprising C₉, C₁₀and heavier aromatics as a bottoms stream in conduit 132. The overheadstream from column 130 may be used at least partially via conduit 230 ofFIG. 2 to generate steam or reboil other columns as discussed previouslyand thus be condensed to provide reflux to the column as well as the netoverhead to xylene separation in conduit 131.

Simultaneously, an isomerized C₈-aromatics stream is passed via conduit165 to high-pressure xylene column 133; this stream contains a lowerconcentration of heavy materials subject to decomposition than the feedto column 130; the column pressure is elevated with respect to that ofthe low-pressure xylene column according to the invention, as discussedpreviously, in order to effect energy savings through concomitantlyhigher temperatures which may be employed to exchange heat at usefullevels. The temperature of the overhead vapor from the high-pressurexylene column 133 therefore is sufficient to provide useful energy toother services in an aromatics complex. As shown, the temperature of theoverhead vapor is sufficient to reboil the low-pressure xylene column130 in reboiler 135 as well as the reboiler of extract column 152, acondensed stream returning as reflux to column 133 (not shown) and a netstream in conduit 136 to xylenes separation. A small net bottoms streamin conduit 138 preferably is sent to low-pressure column 130 forrecovery of remaining C₈ aromatics.

Alternatively or in addition, the temperature of overhead vapor fromhigh-pressure xylene column 133 is sufficient to generate steam usefulfor heating services or to reboil columns in other processing units.Such steam is generated usually at a pressure of in excess of about 300kPa, preferably at least about 500 kPa, and most preferably about 1000kPa or higher. The overhead stream may be indirectly heat exchanged witha water circuit which feeds a steam drum. Most usually, boiler feedwater is heated in heat exchangers decoupled from the steam drum.Multiple water circuits serving different exchangers are arranged inparallel with each other and feed a single steam drum to provide a steamproduct of a desired pressure for which only one set of instrumentationis needed. Such steam systems are well known, and details can be addedthrough such teachings as found in U.S. Pat. No. 7,730,854 which isincorporated herein by reference.

Energy recovery according to the present invention, often involvingclose temperature approaches between process fluids, is improved throughthe use of exchangers having enhanced nucleate boiling surface. Suchenhanced boiling surface can be effected in a variety of ways asdescribed, for example, in U.S. Pat. No. 3,384,154; U.S. Pat. No.3,821,018; U.S. Pat. No. 4,064,914; U.S. Pat. No. 4,060,125; U.S. Pat.No. 3,906,604; U.S. Pat. No. 4,216,826; U.S. Pat. No. 3,454,081; U.S.Pat. No. 4,769,511 and U.S. Pat. No. 5,091,075; all of which areincorporated herein by reference. Such high-flux tubing is particularlysuitable for the exchange of heat between the overhead of the secondhigh-pressure xylene column and the reboiler of the first low-pressurexylene column or for the generation of steam from the xylene-columnoverhead.

Typically, these enhanced nucleate boiling surfaces are incorporated onthe tubes of a shell-and-tube type heat exchanger. These enhanced tubesare made in a variety of different ways which are well known to thoseskilled in the art. For example, such tubes may comprise annular orspiral cavities extending along the tube surface made by mechanicalworking of the tube. Alternately, fins may be provided on the surface.In addition the tubes may be scored to provide ribs, grooves, a porouslayer and the like.

Generally, the more efficient enhanced tubes are those having a porouslayer on the boiling side of the tube. The porous layer can be providedin a number of different ways well known to those skilled in the art.The most efficient of these porous surfaces have what are termedreentrant cavities that trap vapors in cavities of the layer throughrestricted cavity openings. In one such method, as described in U.S.Pat. No. 4,064,914, the porous boiling layer is bonded to one side of athermically conductive wall. An essential characteristic of the poroussurface layer is the interconnected pores of capillary size, some ofwhich communicate with the outer surface. Liquid to be boiled enters thesubsurface cavities through the outer pores and subsurfaceinterconnecting pores, and is heated by the metal forming the walls ofthe cavities. At least part of the liquid is vaporized within the cavityand resulting bubbles grow against the cavity walls. A part thereofeventually emerges from the cavity through the outer pores and thenrises through the liquid film over the porous layer for disengagementinto the gas space over the liquid film. Additional liquid flows intothe cavity from the interconnecting pores and the mechanism iscontinuously repeated. Such an enhanced tube containing a porous boilinglayer is commercially available under the trade name High Flux Tubingmade by UOP, Des Plaines, Ill.

FIG. 4 illustrates examples of specific units within an aromaticscomplex in which direct heat exchange of overhead from one or morehigher-temperature columns to reboilers of one or more lower-temperaturecolumns could achieve energy savings, using numerical designations ofprocesses from FIG. 2. Overhead in conduit 134 from the high-pressurexylene column 133 has a temperature sufficient to provide energy toreboil extract column 152 via reboiler 235, condensing the xyleneoverhead in conduit 236 for return to 133 as reflux or net overhead. Theextract column may be pressurized such that overhead in conduit 256 hasa sufficient temperature to reboil finishing column 155, whichpreferably operates at vacuum pressure, via reboiler 257, condensingextract column overhead in conduit 258. As before, the productpara-xylene is recovered in conduit 156.

FIG. 5 summarizes a number, not exhaustive or exclusive, of directheat-exchange possibilities related to FIG. 2. High-pressure xylenecolumn 133 may provide heat to reboil one or more of low-pressure xylenecolumn 130, extract column 152, and raffinate column 159. Thelow-pressure xylene column 130 may provide heat to reboil extractivedistillation column 120. A pressurized extract column 152 may provideheat to reboil one or more of benzene column 123 and finishing column155. A pressurized raffinate column 159 may provide heat to reboil oneor more of reformate splitter 114, toluene column 126, and deheptanizer164.

FIG. 6 summarizes nonexhaustive examples of indirect heat-exchangepossibilities through the generation of medium-pressure steam. Overheadstreams 230 (FIG. 2) from the low-pressure xylene column 130 and 260(FIG. 2) from the pressurized raffinate column 159 may generatemedium-pressure steam in header 100 at 0.6 to 2 MPa, and preferably 0.7to 1.5 MPa which can be used to reboil one or more of reformate splitter114, extractive distillation column 120 and toluene column 126 with theadded potential of exporting steam to other units. Such generation andusage of steam can be considered as a supplement or substitute for otherenergy savings such as those described in FIG. 5. For example, thehigh-pressure xylene column 133 may provide heat to reboil thelow-pressure xylene column 130 and extract column 152, which in turnreboils the benzene column 123 and finishing column 155.

EXAMPLE

The combination of steam generation and direct heat exchange describedabove in FIG. 6 was evaluated in terms of payback on investment. Thebase case is the facility described in FIG. 1 and the case of theinvention is the FIG. 6 case as applied to the flow scheme in FIG. 3.The relative key parameters for the production of para-xylene are asfollows:

Base Case Invention Fuel consumption 1.0 0.922 Net Steam consumed 1.0 0generated 1.0 1.35

1. A process for producing one or more individual xylene isomers fromfeed streams containing C₈ aromatics and C₉-and-heavier aromatics,comprising: (a) a distillation process comprising two or more xylenecolumns separating C₈-aromatics streams from a C₉-and-heavier aromaticsstreams contained in at least one higher-boiling and at least onelower-boiling feed stream, wherein the at least one higher-boiling feedstream has a higher content of C₉-and-heavier aromatics than the atleast one lower-boiling feed stream, comprising distilling the at leastone higher-boiling feed stream in at least one first low-pressure xylenecolumn to separate a first C₈-aromatics stream from a firstC₉-and-heavier aromatics stream, distilling the at least onelower-boiling feed stream in at least one second xylene column at anelevated pressure to separate a second C₈-aromatics stream from a secondC₉-and-heavier aromatics stream, and condensing an overhead stream fromthe at least one second xylene column by exchanging heat with one ormore of a reboiler of the at least one first low-pressure xylene column;(b) a xylene-isomer separation process to recover one or more xyleneisomers from one or both of the first and second C₈-aromatics streams byinjecting the one or both aromatics streams and a desorbent stream intoa moving-bed adsorption process to obtain a first mixture of the one ormore xylene isomers and desorbent and a second mixture ofnon-equilibrium C₈ aromatics and desorbent; and, (c) adesorbent-recovery process to separate the first mixture of step (b) bydistillation in an extract column with a reboiler having an operatingpressure of at least 300 kPa to obtain the one or more individual xyleneisomers and a desorbent stream returned to the xylene isomer separationprocess, wherein the bottoms stream of the at least one second xylenecolumn of step (a) further exchanges heat to supply a reboiler of theraffinate column.
 2. The process of claim 1 wherein the low pressure isbetween 100 and 800 kPa and the elevated pressure is about 400 kPahigher than the low pressure.
 3. The process of claim 1 wherein thereboiler of the at least one first low pressure xylene column has anenhanced nucleate boiling surface.
 4. The process of claim 1 wherein thelower-boiling stream comprises part or all of a bottoms stream fromdeheptanization of a C₈-aromatics isomerization product.
 5. The processof claim 1 wherein overhead stream from the at least one first lowpressure xylene column exchanges heat with a medium-pressure steamgenerator to generate steam.
 6. The process of claim 1 wherein the oneor more individual xylene isomers includes para-xylene.